Totally unmanned operation is practical in program machining using an NC machine tool, and an operator need not monitor the machining work except for prearranging operations such as mounting of workpieces, starting of machining programs, etc. Thus, while the NC machine tool is executing programmed machining, the operator can perform prearranging operations for the next machining cycle or other odd jobs. If the operator is previously informed of the time of termination of the machining, however, he can more smoothly carry out operations such as removal of machined workpieces, transfer of workpieces to the next machining stage, etc.
Conventionally, in order to know an expected machining termination time, the operator used to compute a required machining time manually in accordance with the machining shape, set machining speed, etc. given by a machining program, thereby estimating the time when the machining would terminate. However, when one and the same portion of a workpiece has to be finished, for example, by repeating several machining cycles ranging from rough machining to finish machining, machining conditions naturally vary with the individual cycles, so that the average speed of tool movement for each cycle, tool offset value for each cycle, and, therefore, tool movement path length for each cycle (overall length of machining locus for each machining stage) vary. Accordingly, the machining time varies with the individual machining stages. Thus, in order to estimate the necessary machining time for any one portion of workpiece in advance, various conditions such as the machining frequency, machining conditions for each machining stage, tool offset value, etc., besides the machining shape given by the machining program, should be taken into consideration, and an accurate estimation usually requires complicated calculations.
Some NC machine tools are designed for being capable of adaptively controlling the machine speed in order to obtain uniform work surfaces. In the cases of the NC machine tools with such arrangement, however, the set machining speed does not always agree with the actual machining speed. Even though complicated manual calculations are made in consideration of the offset value and the like, the required time calculated not based on the actual machining speed but based on the set machining speed may lack in accuracy in some cases.